This invention relates to a directional information system where a number of input signals are encoded for recording or transmission on a medium into two or more channel signals and where the channel signals are decoded into a number of output signals corresponding to the directional information input signals. The decoder of this invention decodes the two or more channel signals so that directional effects are enhanced.
In quadraphony the loudspeakers are spaced horizontally around the listeners in four locations, to create an impression of the original program in full horizontal surround sound. In some quadraphonic systems the loudspeakers are placed at the four corners of the room. In other quadraphonic systems, such as those used in motion picture theaters, loudspeakers are not all placed at corners. Instead they may be placed at the left and right front corners of the theater, at the center of the front stage and dispersed around the back wall of the theater. The loudspeakers placed at the front left and right corners are still known as the left and right speakers; the ones placed at the center of the front stage known as the center speakers; and those at the back wall as the surround speakers. In order for the recording played back through the loudspeakers to recreate a realistic impression of the original program, the recording must contain directional information. In some quadraphonic systems four discrete input channels are actually recorded; this is known as the 4-4-4 format. The other general approach, termed 4-2-4, uses some kind of matrix encoding of the four audio input channels into two channels such as two conventional stereo-recorded channels, which are decoded back to four audio output channels during playback.
In the 4-2-4 sound systems, since the four directional audio input signals are transformed into two channel signals by the encoder, some directional information will be lost so that it is impossible for the decoder to reproduce signals perfectly identical to the original directional audio input signals. As a result, the cross-talk between adjacent channels and the reproduced sound signal may greatly reduce the directional effect of the quadraphonic system.
Numerous attempts have been made to enhance the directional effects of quadraphonic 4-2-4 systems. In one approach known as gain riding, the net sound level of each of the four loudspeakers is adjusted without adjusting the relative contributions of the two channel signals to reduce cross-talk. In another approach known as the variable matrix approach, the four output signals fed to the four loudspeakers are derived by certain mathematical computations performed on the two channel signals to vary the relative contributions of the two channel signals in order to reduce the effect of cross-talk.
Ito et al., in U.S. Pat. No. 3,825,684, disclosed a variable matrix decoder for enhancing the directional effects of a four channel playback system with loudspeakers placed at the four corners of the room. The decoder has a control unit which detects the phase difference between the two channel signals and produces two control signals, one for controlling the separation of the two front outputs and the second control signal for controlling the separation of the two rear outputs. The two control signals are also used to control the level of the front output signals relative to the rear output signals. In reference to FIG. 10 of U.S. Pat. No. 3,825,684, for example, the separation between the two front outputs is controlled by the gain f applied by variable amplifier 122 and appears to vary inversely with the magnitude of the phase difference between the two channel signals L and R. The separation between the two rear outputs is controlled by the gain b of variable amplifier 127 and appears to vary directly with the magnitude of the phase between L and R.
In U.S. Pat. No. 3,944,735, Willcocks discloses a directional enhancement system used together with existing matrix decoders and for enhancing the directional effects of output signals from these decoders. It does not include a 2-4 matrix decoder as such. Instead the system modifies the four output signals obtained from a preceding quadraphonic matrix decoder to enhance the directional content of the signals before presenting them to the loudspeakers. The system comprises a detector which generates 6, 8 or 10 directional control signals by comparing envelopes of certain signals derived by fixed matrices from the channel signals. The detector generates these control signals using automatic gain control to avoid dependence on signal levels. Willcocks employs a processor which generates from the control signals the coefficients of a modifying matrix, and employs a matrix modifier which modifies the four output signals of the preceding matrix decoder by the modifying matrix.
None of the above directional enhancement systems for 4-2-4 quadraphonic decoders is entirely satisfactory. It is therefore desirable to provide systems with better directional enhancement capabilities and with simpler circuitry.